In the past a major problem encountered in attempting to make realistic recordings or accurate acoustical measurements has been the discrimination by the microphone between the frequency spectra of the direct and random incidence (diffuse field) acoustical pressure variations. This discrimination is caused by the physical dimensions of the microphone as compared to the shortest wavelength of interest. Since even a microphone with a diaphragm as small as 0.5 inches (1.27 cm) in diameter will appear as an obstruction to a plane wave acoustical radiation because its diameter is one-half the wavelength at 13,560 Hertz, a pressure build-up will occur at the face of the diaphragm to zero degree incidence, acoustical pressure variations beginning at frequencies lower than 13,560 Hertz. This causes the pressure to increase at these higher frequencies and therefore a greater electrical output is produced at the microphone output terminals. This effect is shown in FIG. 1 in curve 1A. This build-up of pressure at the microphone diaphragm does not, however, occur for random incidence (diffuse field) acoustical pressure variations; see FIG. 1, curve 1B. Therefore, these is caused to be discrimination between zero degree, incidence and the random incident (diffuse field) acoustical pressure variations, as a result of physical presence of the microphone in the acoustical field it is attempting to sample.
To overcome the pressure increase at higher frequencies due to a microphone's physical dimensions, microphone designers have taken this into account and have designed conventional microphones to produce a flat characteristic response for zero degree incidence plane acoustical waves; see FIG. 1, curve 1C. But the response of such microphones to random incidence acoustical signals necessarily shows a decrease in output at the higher frequencies; see FIG. 1, curve 1D. Conventional microphones are designed to be used with their diaphragms or major entry ports aimed directly toward, or in the general direction of, the source. Since these conventional microphones discriminate between the zero degree incidence and random incidence acoustical signals, the direct sound and random incidence sound will exhibit different output versus frequency characteristics. This means that an unnatural spectral balance between acoustical signals arriving at the microphone from different directions, or between the direct and reverberant sound, will exist in the electrical output from such microphones; see FIG. 2, curves A through D.
FIG. 2A shows the amplitude vs. frequency response of a normal free field type microphone for zero degree incidence, plane wave sound. FIG. 2B shows the amplitude vs. frequency response of this same microphone for random incidence or reverberant sound. FIG. 2C is the amplitude vs. frequency characteristic for a typical reverberant environment and is included only for completeness in describing all of the factors which contribute to the spectral balance of the composite sound. FIG. 2D is the spectral balance of the final, composite sound as picked up and transduced into an electrical analog signal by a conventional free field type microphone, and is the result of the combination of effects shown in FIGS. 2A, B, and C. The effect upon the final spectral balance of the signal caused by the environment and shown in FIG. 2C is not part of the problem since it exists in the natural environment. It is not an object of this invention to place any restriction on the effects upon the spectral balance caused by the natural environment. It is an object of this invention, however, to allow the effects of the environment upon the spectral balance of the acoustical signal at the microphone position, to be transduced accurately and without the ambiguity heretofore caused by previous microphone techniques or methods.
It is an object of this invention to provide a process and microphone arrangement which overcomes this problem, by causing the microphone to respond without discrimination, with respect to its spectral content, to the direct and random incidence sounds arriving at the position of the microphone within the frequency range of interest.
It is also an object of this invention to provide a process and microphone arrangement which allows acoustical filtering in order to cancel undesired signals whose frequencies are above the band of interest. This acoustical filtering is caused by the interaction of the direct and reflected signals and therefore a sharp cut-off characteristic is achieved without the deleterious phase shift and non-uniform group delay of the equivalent high-order electrical wave filters.
It also eliminates the adverse effect upon spectral balance, due to the cancellation and addition effects, caused by the reflections from the adjacent boundary, proximate to the microphone, when using conventional microphone placements. Because such conventional microphone placements are usually at a moderate distance from boundaries, wavelength-dependent cancellations and additions will result from the interaction between the direct sound and the reflected sound from the nearby boundary. Placing a microphone close to a boundary with its diaphragm, or major entry port, perpendicular to the boundary does not completely ameliorate this latter problem since it merely moves the frequency of the first cancellation to a higher frequency, which is still within the audible spectrum. Such placement results in an uneven distribution of pressure across the diaphragm, which is wavelength-dependent; see FIG. 3. In FIG. 3, responses F1, G1, and H1 are computer-derived results for the combination of the direct and reflected sound at 1/16", 5/16", and 9/16" above the boundary respectively. An actual measured response is also shown which is the composite result of the combination of the direct and reflected sound at all distances between 1/16" and 9/16" above the boundary.
It is an object of this invention to allow the frequency of the first cancellation to be moved to an upper frequency, by controlling the distance between the boundary and the diaphragm or major entry port, which is determined by the process formula. At least two other benefits are derived from the use of the invention, which are (1) an increase in the practical sensitivity of the microphone approaching the theoretical maximum improvement of 6 dB, and (2) an increase in the practical signal-to-noise ratio approaching the theoretical maximum improvement of 6 dB.